The invention relates to a rotatable chuck for glass tubes, especially for machines for the production of vials or ampules made from glass tubes.
A vial machine, in which vials or ampules are produced from glass tubes by rolling, utilizes rotatable chucks spaced at regular intervals and usually carried on a turntable, from which glass tubes are successively fed to various processing stations. The chucks are rotatable, so that axially symmetrical shaping of the vial is possible. The shaping steps performed are in this case normally rolling of a mouth area and smoothing of the bottom on the vial. Since shaping of the glass tube is possible only at elevated temperatures (550.degree.-1,000.degree. C.), the chucks have to operate reliably even at comparatively high temperatures. On the other hand, gentle grasping and holding must be possible with the chuck so that the glass tube held by the chuck is not destroyed.
A chuck of the above-described type is known from U.S. Pat. No. 4,832,726. This chuck described exhibits three clamping jaws, placed at regular spacing intervals around a tube, which jaws are guided in inclined guides. The ends of the clamping jaws facing away from the glass tube to be held each include a ball bearing, which in every case is guided in a thrust ring. A spring acts on the thrust rings, so that the clamping jaws in the inclined guides are moved at an acute angle in the direction of the longitudinal axis of the glass tube. The thrust ring is retractable against the spring pressure by an actuating lever, by which lever the clamping jaws guided by the ball bearing in the thrust ring are retracted from the glass tube. This chuck has a comparatively large linear extension and can securely grasp glass tubes only within a certain diameter range. If glass tubes of another diameter range are used in these chucks, they have to be changed, i.e., other clamping jaws and/or another spring are used in the chuck.
For this purpose, it is necessary to provide for various clamping jaws and/or springs, and further, an additional expenditure of time is added because of the changeover.